theorem
  S is (R1) (R3) (R11) implies a * (a" * b) <<>> b
  proof
    assume
A1: S is (R1) (R3) (R11);
    take (a * a") * b;
    thus (a * a") * b =*=> a * (a" * b) by A1,Th2;
    (a * a") * b ==> 1.S * b & 1.S * b ==> b by A1,ThI2;
    hence (a * a") * b =*=> b by Lem3;
  end;
